The present invention relates to a secondary battery which employs zinc as a negative-electrode material.
Batteries are widely used as a power source for electronic and electric apparatuses. The recent trends towards miniaturization, higher performance, portability, and personal use of electronic and electric apparatuses have brought about increased demand for secondary batteries which are suitable for long term use and yet are economical.
For use in technical fields, nickel-cadmium and alkali-zinc secondary batteries have been investigated. In particular, alkali-zinc secondary batteries which employ zinc as the negative-electrode material have the advantages of high energy density and economically low cost.
However, secondary batteries employing zinc as the negative-electrode material have disadvantages, including deterioration of the zinc electrode by repetition of charge and discharge and short cycle life.
The causes of the deterioration of the characteristics of the zinc electrode with the repetition of charge and discharge are considered as below. The zinc electrode tends to dissolve unevenly during discharge because of delicate non-uniformity of the surface state, so that the surface roughness of the zinc electrode will increase with the progress of the discharge, and the zinc electrode will come to be deformed. During charge, zinc deposits in a form of a tree-branch (which is called a dendrite) on the projected portion. The dendrite will grow toward the positive electrode with the repetition of charge-discharge cycles, the deposited zinc finally penetrating the separator to cause internal short-circuit, or dendrite short-circuit.